In PWM amplifier power stages utilizing IGBT (insulated gate bipolar transistor) or FET (field effect transistor) power switching devices, the switching transistors are usually connected in pairs in totem pole fashion between the rails of the power source. The output load terminal can be connected to the upper rail through the upper switching transistor or can be connected to the lower rail through the lower switching transistor. The power supplied to the load is controlled according to the pulse width determined by the ON time during each operating cycle. Most motor controllers are of a three phase design and therefore include three pairs of power switching transistors.
Such power stages generally require a floating power supply to bias the upper switching devices into the conductive or ON state. A "bootstrap" capacitor can be employed for this purpose. The bootstrap capacitor is charged while the lower switching transistor is conductive and connects the capacitor to the lower rail. When the lower transistor is OFF and the upper switching transistor is being rendered conductive, the capacitor is level shifted to the upper rail and drives the upper switching transistor into the fully conductive state. With this arrangement a portion of each operating cycle must be reserved for recharging the capacitor and, therefor, the lower switching device must be ON for a minimum portion of each operating cycle regardless of the instantaneous power needs. As a result the duty cycle is limited to about 85% and only about 70% of the available power can be supplied to the load.